In wheel slippage control systems (ABS or ASR) it is known to count during a computing cycle the nulls of the output voltage of the sensors measuring wheel speed and from this count and the time elapsed since the first null to compute the speed of the wheel. The number of nulls to be counted has been limited to a maximum (e.g., 16 in a computing cycle of 10 msec), in order to minimize the load on the microprocessor by program interrupts and thus keep the computing cycle down to 10 ms. The (last) computed speed is then limited, in some cases, (e.g., to 50 g change in both directions) and stored, and used in the next computing cycle as the speed value to obtain the actuating signals for brake pressure control by means of a preset algorithm. At high rotatory wheel speeds a speed value that is relatively remote in time from the new computing cycle is used for the control.
It is also known (DE-C2 2801520 FIG. 2) to count the successive half-waves of the sensor signal separated by pulses of higher frequency, store the partial measured value first obtained and, after the second partial measured value is obtained, to determine by totalization the period length that is used for the control.